Recently, electronic equipment has been miniaturized. The development of personal digital assistants and mobile computers has accelerated the miniaturization demand, and thus multilayer wiring boards to be packaged in such electronic equipment are required to have further functions, be thinner and have fine wiring patterns.
Electronic equipment such as communications equipment is required to operate at high speed. Therefore, multilayer printed boards should also correspond to the high-speed operation. For this purpose, the wiring length is reduced, and at the same time, wiring width and pitches are decreased so as to shorten the time for conveying electric signals. That is, a multilayer board should be small, thin and densified.
For proceeding with high-density packaging of multilayer boards, formation of fine wiring pattern is emphasized. In a typical printed wiring board, for example, a subtractive method of forming a wiring pattern is known. In the subtractive method, a copper foil having a thickness of about 18 μm to about 35 μm is chemically etched to form a wiring pattern on a substrate. It is said that even this method has a difficulty in volume-producing a wiring pattern having a line width of 75 μm or less. For providing a finer wiring pattern, the thickness of the copper foil should be decreased.
Since a wiring pattern formed by the subtractive method protrudes from the substrate surface, solder or a conductive adhesive for electric connection is difficult to dispose on a bump formed on a semiconductor. The bump may shift to a space between wiring patterns and cause a short circuit. The protruding wiring pattern may be an obstacle in a subsequent step of sealing with resin.
For example, JP10(1998)-84186 A describes a method of forming previously a fine wiring pattern on a separable film, and transferring only the wiring pattern to a desired substrate.
For securing transfer performance of the wiring pattern, JP10(1998)-51108 A or the like discloses a method of previously heating a separable film so as to prevent dimensional error caused by shrinkage of the separable film at transferring.
For securing transfer performance of the wiring pattern similar to the above JP10-51108 A, JP2002-111203 A or the like discloses a method of forming a metal foil on a separable film via an adhesive layer, and adjusting the tensile strength of the metal foil so as to manufacture a reliable wiring board with a good yield. The wiring board hardly causes any positional deviation at the time of transferring.
JP 2002-359455 A or the like describes use of silicone for an adhesive layer provided on a separable film as in the above-described transferring, since silicone is not aged, and it is superior in chemical resistance. This technique provides excellent patterning retention at the time of wiring formation by etching or the like.
This technique is useful in avoiding a problem of protrusion of a wiring pattern, since a substrate with a wiring pattern embedded thereon has a planar surface.
However, when transferring a fine wiring by this method, dimensional errors will occur among the transferred wirings when a cost-effective polyethylene terephthalate (PET) or the like is used, and this causes connection failures with vias or with a semiconductor bump. For addressing this problem, JP10-51108 A, JP2002-111203 A and JP2002-359455 A disclose techniques for correcting the dimensional change or for suppressing the shrinkage caused by heat at transferring to be 0.05% or less by previously heating the separable film.
Since relatively high heat is applied at the time of transferring the wiring pattern onto a substrate by using a separable film (for example, temperature of 80° C. to 170° C. is described), a conductive paste often can be cured at a time of transferring a wiring pattern on a prepreg sheet in a stage ‘B’ having viaholes filled with the conductive paste.
In the conventional technique as mentioned above, a plurality of prepreg sheets (wiring patterns are embedded therein) with a cured conductive paste are laminated and subjected to heat and pressure so as to cure the plural prepreg sheets in a batch, thereby manufacturing a multilayer wiring board. Problems in the manufacturing method are, for example, non-uniform pressure can be applied to the sheets or close contact with the wiring pattern can deteriorate due to the cured conductive paste at the time of curing the laminated sheets in a batch, which will cause disadvantages in obtaining a via-connection.
That is, a package using the conventional transferring material has difficulty in satisfying all of the requirements, that is, the transfer performance, the dimensional stability, and the via-connection reliability.